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机械类外文翻译之-平面度测量Procedure for performing flatness measurementFlatness measurements are performed to check the flatness of CMM tables and surface plates. It determines whether any significant peaks or troughs exist and quantifies them. If these errors are significant then remedial work, such as lapping, may be required.To understand the basic principles and techniques of flatness measurement refer to:The principles of flatness measurement.Standard methods of assessing flatness.Principle of flatness measurementThe flatness measurement kit is used to measure flatness. The angular interferometer is attached to the turning mirror and the angular reflector is attached on top of the selected flatness base. The angular interferometer is placed in the path between the laser head and the angular reflector.Figure 1 - Principle of measurementThe laser beam is split into two by the beam-splitter inside the angular interferometer. One part of the beam (the measurement beam A1) passes straight through the interferometer and is reflected by one of the twin reflectors of the angular reflector back through the interferometer and into the laser head. The other beam (measurement beam A2) passes through the periscope part of the angular interferometer to the second reflector from where it returns through the interferometer and into the laser head.?An angular measurement is produced by comparing the path difference between the beams A1 and A2, (i.e. the measurement is independent of the distance between the laser and the interferometer). The flatness reading displayed by the software is the incremental height between the front and back feet of the flatness baseplate on which the angular reflector is fitted. This incremental height is calculated from the angular measurement and knowledge of the distance between the centres of the front and back feet of the flatness baseplate. This distance, termed the foot-spacing, must be entered into the calibration software before measurement starts.?For each measurement line (see Standard methods for assessing flatness), the angular interferometer (mounted on the flatness turning mirror) stays stationary, while the reflector (mounted on a flatness base) moves along the line in incremental steps defined by the foot-spacing.A flatness measurement is carried out by taking a series of incremental height readings as the angular reflector is moved along the measurement path. Figure 2 - Incremental measurements drawing changes In Figure 2 above: Position I is the initial position at which the interferometer reading is normally datumed. Position II is one foot-spacing beyond Position I. The interferometer reading will be the incremental distance d1, which is the difference in height (with respect to the datum line) of the front and back feet of the flatness baseplate. Position III is one foot-spacing beyond Position II. The interferometer reading will be d2, i.e. the difference in height between the front and back feet of the baseplate in its new position. Similarly, the reading at Position IV will be d3, and so on for all subsequent positions on the measurement line. The actual flatness of the measurement line will be the algebraic sum of the reading d1, d2 etc (plus the reading at the datum position (I) if it had not been zeroed). Note: Environmental compensation is not required when taking flatness measurements, as the difference in path lengths between the two beams is so small that the error due to environmental effects is negligible.Standard methods for assessing flatnessTo measure the flatness of a surface, a number of measurement lines need to be taken over the surface. There are two standard methods of carrying out flatness measurements:Moody methodGrid methodThe flatness of a surface can be defined as the separation of two planes which are parallel to the general trajectory of the surface and which just enclose the measured surface as shown in Figure 1.Figure 1 - Flatness specificationFlatness deviations for a surface plate of a known size can be compared with permitted deviations in national standards.Moody methodWith the Moody method, measurement is restricted to the eight prescribed lines as shown in Figure 2.Figure 2 - Moody map of surface plate The Moody method was first proposed by J.C. Moody in 1955 and has subsequently achieved wide acceptance. The method provides a relatively quick method of calibrating a surface plate, with the results being presented as a contour plot along the eight measurement lines tested, in a format acceptable for certification.This method does have one disadvantage - all of the points on all of the eight lines must be measured and plotted. This can cause problems in defining a foot-spacing which will meet this requirement, particularly on slotted tables/surfaces where the position of one or more slots may coincide with the required position of one of the feet of the flatness base.Grid and half grid methodWith the Grid method, any number of lines may be taken in two orthogonal directions across the surface as shown in Figure 3. Figure 3 - Grid map of surface plateWith the Grid method, whilst the incremental nature of the measurement technique requires that all points on a given line are measured, it is not necessary to take measurements on all lines. This allows the measurement lines to be configured to avoid obstructions (e.g. slots) or to provide greater detail in a given area.The Half-grid method as shown in Figure 4 is a special case of the grid method where a number of measurement lines are taken in one direction (e.g. the X-axis), but only the perimeter lines are used for the orthogonal direction.Figure 4 - Half-grid map of surface plateA disadvantage with both grid methods is that they require a reference plane to be defined. An alternative technique such as the Moody method is needed to define this reference plane.To perform flatness measurements using the Moody method, carry out the following steps:Prepare the surface plate for flatness measurement.Set up the?flatness data capture software.Set up the laser and optics.Align the laser beam along a measurement line.Capture data for the measurement line.Set up and measure the other seven measurement lines.Analyse the captured flatness data. Inspect the data for measurement errors as discussed in Factors affecting accuracy of flatness measurementPreparation of the surface plateBefore calibration, the surface plate should be cleaned thoroughly and left to reach thermal equilibrium with the surroundings. A map of the measurement lines should be marked on the surface plate itself with lead pencil or chalk.All markings made on the surface plate should be offset from the measurement line by either 8.5?mm (0.33?in) or 41.5?mm (16.33?in), these being the distances from the longitudinal edges of the baseplates to the centre line through the front and back feet as shown in Figure?1. Care should be taken not to mark the table where a flatness foot placing will occur, as this may cause measurement errors. Figure 1 - Location of perimeter linesTo allow the placement of the turning mirror, the perimeter measurement lines should be at least 108?mm (4.25?in) from the edge of the surface plate on the side that will be parallel to the laser beam, and at least 75?mm (3?in) from the other three edges as shown in Figure 1.The length of each measurement line should be an integral multiple of the foot-spacing selected. This places a constraint on the surface area which can be measured and it may not be possible to map the whole of the surface area. When using the Moody method, the constraint is more severe as this requirement applies to the two diagonals as well as the orthogonal axes. In practice, acceptable closure errors can normally be achieved if the length of the diagonal meets this requirement to within 1% of the foot-spacing. The Renishaw data capture software provides assistance in selecting a suitable number of target points (and hence an effective table size) for any given foot-spacing and table size.Flatness data capture softwareSet-up of laser and optics1. Mount the ML10 laser on the tripod remote from the surface plate. Ensure the laser is level. Once positioned, the laser should not be moved.2. Align the laser head parallel to the surface plate and to a perimeter line, with the beam at least 83?mm (3.25?in) from the perimeter line and 25?mm (1?in) from the edge of the surface plate as shown in Figure 2.3. So that all measurement lines can be taken without moving the ML10 laser, mount the angular interferometer, as shown in Figure 3, on the base of a flatness turning mirror. Mount the angular interferometer so that the side with the single optical aperture is facing the mirror and the side with two optical apertures is facing the angular reflector. This is used for all measurement lines. A second flatness turning mirror is used for those measurement lines which require a further deviation of the beam, see?Figures 4, 5, 6, 7 and 11.Figure 3 Measurement line set-up and capturePerform a measurement along each of the measurement lines as shown below:? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l EA#EA Diagonal EA? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l CA#CA Perimeter line CA? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l DH#DH Centre line DH? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l EG#EG Perimeter line EG? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l AG#AG Perimeter line AG? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l BF#BF Centre line BF? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l CE#CE Perimeter line CE? HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Preparation%20and%20set-up.htm l CG#CG Diagonal GC?For each measurement line, align the optics over the entire length of travel?Capture data for that measurement line.Minimise measurement errors as discussed in HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Factors_flatness.htm factors affecting accuracy of flatness measurements.Moody measurement line configurationsFigure 4 - Typical Moody measurement routine - diagonal EAFigure 5 - Typical Moody measurement routine - perimeter line CAFigure 6 - Typical Moody measurement routine - centre line DHFigure 7 - Typical Moody measurement routine - perimeter line EGFigure 8 - Typical Moody measurement routine - perimeter line AGFigure 9 - Typical Moody measurement routine - centre line BFFigure 10 - Typical Moody measurement routine - perimeter line CEFigure 11 - Typical Moody measurement routine - diagonal GCFlatness analysisThe Laser10 analysis software includes facilities for plotting the data to either Moody or Grid format.Analysis softwareThe analysis software can be accessed by selecting Data/Analyse from the menu bar or by clicking on the button on the toolbar of the flatness data capture software.Now select the Analysis option from the menu bar and the following options are available: Moody or Grid plot Print numeric Moody or Grid data Whether the Moody or Grid options are available depends on the data in the file loaded. In the following example, Moody has been selected.Figure 1 - Moody flatness contour plotThe control panel overlaid on top of the plot allows you to select the line and point that you want to analyse. Use the arrow keys to select the position that you want to analyse. The flatness error in micrometres is shown for the selected line and point. When you click on the Done key, the control panel is hidden.Both Moody and Grid presentations include a characteristic statement of flatness range in the box below the three-dimensional projection of the surface. The range defines flatness of the measured surface as the separation of the planes which just enclose the measured surface as discussed in standard methods of assessing flatness.The closure errors are also defined.If you had selected Grid, the display would be as follows:Figure 2 - Grid flatness contour plotPlot optionsThe Plot/Elevation and Rotation option on the menu bar allows you to alter the projection of the plot. When you select this you will see the following:You can now change the elevation and rotation by dragging the pointers. The effect can be seen by comparing Figure 3 (where the elevation and rotation are 45o and 75o respectively) with Figure 1 (where they are both 35o).Figure 3 - Moody plot - alternative projectionThe Plot/Units option on the menu bar allows you to alter the units being used. When you select this, you will see the following:Use the mouse to select the units you want to use then click OK.Closure errorsTypical flatness contour plots are shown in Figures 1 and 2. The closure errors reported are an indication of the validity of the data.Figure 1 - Moody flatness contour plotOn the Moody plot, there are two closure errors, corresponding to the differences in readings between the intersection of the diagonals and lines FB and HD. These are known as closure errors 7 and 8 respectively.Figure 2 - Grid flatness contour plotOn the Grid plot, four reference points are defined, and the X and Y lines through these points are reference lines. There will be n closure errors, corresponding to the number of intersections between two non-reference lines. Only the maximum value is displayed.With a Half-Grid plot there are no closure errors, since there are only two Y lines, both of which are reference lines.The maximum value of the closure errors gives an indication of the validity of the calibration. If the worst closure error is less than 2.5 HYPERLINK JavaScript:popup.TextPopup(um,popfont,9,9,-1,-1) o Click for pop-up definition m, the calibration can be considered to have been completed satisfactorily. If any closure error is greater than this value, the measurements and analysis may have to be repeated.Factors affecting accuracy of flatness measurementsThe following factors can affect the accuracy of measurement:Beam alignment errorAngular optics errorDifferential expansion of flatness base feetRepeatability of measurementsBeam alignment errorAs discussed in HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/Beam%20alignment%20procedure.htm Beam alignment procedure, the error in alignment between the laser beam and the measurement line will affect the accuracy of measurement. The maximum alignment errors allowed to achieve the measurement accuracies given in the HYPERLINK ms-its:reference.chm:/specifications.htm l flatness Specifications section are shown below:Flatness base Maximum alignment error50? HYPERLINK JavaScript:popup.TextPopup(mm,popfont,9,9,-1,-1) o Click for pop-up definition mm 100?mm150?mm81 HYPERLINK JavaScript:popup.TextPopup(arcmin,popfont,9,9,-1,-1) o Click for pop-up definition arc min 54 arc min27 arc minThese alignments should be achievable by eye.?Note: An alignment error of 1.5 arc min is equivalent to a 0.5?mm error over 1? HYPERLINK JavaScript:popup.TextPopup(m,popfont,9,9,-1,-1) o Click for pop-up definition m.Differential expansion of flatness base feetDifferential expansion of the front and back feet of the flatness base will cause an error. As with the angular reflector housing, the flatness base should be handled as little as possible during a measurement and should be kept away from heated or cooled surfaces or air masses so that its temperature remains stable.Repeatability of measurementsAny accumulation of dirt particles or granite dust under the feet of the flatness? base can introduce a measurement error. Measurement errors are often signified by a large HYPERLINK mk:MSITStore:D:毕业设计新建文件夹毕设flatness.chm:/closure.htm closure error. To reduce measurement errors to a minimum, the reflector should be moved to the centre of the line and the laser system zeroed. The reflector should then be moved from end to end along the measurement line several times, while the readings from the laser read-out are observed. These readings should be repeatable to within 0.5? HYPERLINK JavaScript:popup.TextPopup(um,popfont,9,9,-1,-1) o Click for pop-up definition 祄 (0.2 HYPERLINK JavaScript:popup.TextPopup(thou,popfont,9,9,-1,-1) o Click for pop-up definition thou) on a clean and good quality surface.平面度测量平面度测量步骤平面度测量是用于检查CMM工作台和划线台的平面度。它测定平面是否存在明显的突起或者凹槽并进行量化。如果这些存在明显的误差，则可能必须进行如刮研等的补救工作。了解平面度测量的基本原理和方法请参考：1,平面度测量的原理2,评定平面度的标准方法平面度测量的原理 使用平面度测量镜组进行测量。其中角度干涉镜（Angular interferometer）是和转向镜（Turning mirror）连接在一起，角度反射镜（Angular reflector）连接在选定的平面度底座（Flatness base）上部。角度干涉镜放置在激光头和角度反射镜路径之间。图一：测量原理图激光束经角度干涉仪内部的分光镜分为两道。其中一道光束（测量光束A1）直接通过干涉仪并经角度反射器中的一个反射镜反射回干涉仪并回到激光头。另一道光束（测量光束A2）通过角度干涉仪的展望镜射向第二个反射镜并经干涉镜返回激光光。角度测量通过比较光束A1和A2的光路差别（测量结果与激光头和干涉镜的距离无关）。软件显示的“平面度”读数是由安装在平面度底座上的角度反射镜前、后点的增量高度。这个增量高度由角度测量和平面度底座前后点中心点距离计算。这个距离，称为点距，必须在测量开始前输入到校准软件中。对于每一道测量线（见平面度测量的标准方法），角度干涉镜（安装在平面度转向镜）保持静止，而反射镜（安装在平面度底座）沿着检测路径以根据点距定义的增量脉冲移动。平面度测量是通过采集一系列当角度反射镜沿着检测路径移动的增量高度读数实现的。图2：图示测量增量变化如上图2示位置I为开始位置，在该点处干涉仪读数为正常值位置II与位置I有一个点距距离的位置。干涉仪读数将增加一个d1距离，该距离表示平面度底座“前”、“后”点之间的高度差（沿着数据线方向）位置III与位置II有一个点距距离的位置。干涉仪的读数将变为d2.举例说明为平面度底座在它新的位置上“前”、“后”点之间的高度差。同理，在位置读数将为d3,其他沿测量路径的所有顺序位置也是如此。测量路径的实际平面度将是读数d1、d2（加上数据位置点I的读数，如果没有则为零）的代数总和。注意：当进行平面度测量时，没有需要进行环境补偿，因为两道光束间在路径方向上的偏差很小，因而由环境引起的误差可以被忽略。平面度测量的标准方法为测量一个表面的平面度，必须在表面上定义一系列的测量路径（measurement line）。平面度测量有两种标准方法：穆迪法栅格法表面的平面度可以定义为两个包围着被检测表面的相互平行平面到表面公共轨线（general trajectory）的偏差值。如图1所示。图1：平面度的定义一个已知表面的平面底偏差可以和国际标准中的允差进行比较穆迪法使用穆迪法，测量被限制在如图2所示的八条指定检测路径图2：平面的穆迪图穆迪法首次由J.C.Moody在1955年提出并随后获得广大的认同。该方法提供了一个相对快捷的方式检测一个平面，检测结果表示为沿着八条检测路径的轮廓线。是一种可被接受的检测方式。这种方法有一个缺点在所有的八条检测线上的所有点都必须进行测量并标定（plotted）。这样就给定义一个满足该要求的点距带来困难，尤其是在狭小的台面或是表面，槽（slots）的位置可能和平面底座所要求的某个点距重叠。栅格和半栅格法使用栅格法，任意数量的路径将被分为如图3所示覆盖整个表面的两个正交方向。图3：表面的栅格图使用栅格法，该测量方法的增量原则要求特定路径上的所有点都必须进行测量，因而允许对测量路径进行修改以避免障碍（例如：槽）或在特定区域提供更多的细节。如图4所示的半栅格法是当在一个方向上（如X轴）进行一系列的测量时，栅格法中的一种特例，但在直角方向上只有轮廓线可使用该方法。图4：表面的半栅格图两种栅格法的共有缺点是它们要求定义一个参考平面。而在其他方法如穆迪未予是不需要对参考平面进行定义的使用穆迪进行平面度测量，执行如下步骤：平面度测量前的工作台准备工作设置平面度数据采集软件安装激光头及光镜平面度光束准直步骤数据采集对其他七个测量路径的安装及测量分析采集的平面度数据。根据影响平面度测量精确度的因素一节检查数据中的测量误差平面度测量准备及设置该节讨论的是使用穆迪法进行平面度测量的准备工作。相似的方法也可用于栅格法。平面工作台准备1.测量开始前，就全面清洁工作台并停放，与便达到和环境相平衡的温度。使用铅笔或粉笔在工作台表面上标示出测量路径“图”。2.在平面工作台上的标示应和测量路径有8.5mm（0.33in）或41.5 mm（16.33in）的偏离，作为如图1所示的底座纵向边界通过前、后点到中线的距离。注意不要在工作台上标示平面度点所经过的点，这样将可能产生测量误差。图1.预置边界线（perimeter line）的位置3.如图1所示，为方便转向镜的安装，预置边界线在和激光束平等的一边应最少距离平面工作台的边界108mm（4.25in），最少距离其他三条边界75mm（3in）.4.每个测量路径的长度就是所选点距的整数倍。这就对可测量的工作台面积进行的限定，有可能不能够对表面的全部面积进行标示。当使用穆迪法时，该限定将因为要同时应用于两个对角轴线及直角轴线上而要求更加严格。在实际使用中，如果对角轴线的限定值小于点距的1%，则通常可以取得较满意的闭合误差。雷尼绍数据采集软件可以对任意给定点距和工作台尺寸选择适合的目标点个数（即有郊的工作台面积）提供的支持。激光头及光镜的安装 1.在远离平面工作台的位置将激光头安装在三角架上。确定激光头是水平的，定位后不要移动激光头。2.对齐激光头和平面工作台及预置边界线平行，激光束就最少距离预置边界线831 mm（3.25in）及距离平面